diff --git a/documentation/agent-notes.md b/documentation/agent-notes.md
index 5b1febb..79804a2 100644
--- a/documentation/agent-notes.md
+++ b/documentation/agent-notes.md
@@ -2304,6 +2304,13 @@ into `.conformance-artifacts/`, which `.gitignore` covers, and
 IGNORED rather than merely uncommitted. A set comparison of `git status --porcelain` alone passes
 happily over a visible working directory; that is why the second half of that test exists.
 
+**THAT TEST RUNS ALONE, AFTER EVERY OTHER TEST FILE.** Vitest runs files in parallel, and three of
+them write into the working tree for a moment: the README usage case, the `tsup` build it spawns,
+and the phi-scan override-gate case. A `git status` taken beside them compares their leftovers, not
+the run's, and the release Verify step on `d906bda` went red that way. `vitest.config.ts` gives the
+file a project of its own with a later `sequence.groupOrder` and says why there. Do not answer a red
+from it by loosening the set comparison.
+
 **SIZE IS THE SECOND REASON AND IT IS INDEPENDENT OF THE FIRST.** The Schematron is about 1 MB and
 its vocabulary file is about 65 MB. Neither belongs in an npm package, and the corpus is CC0 so
 licence is not what forbids vendoring it.
diff --git a/vitest.config.ts b/vitest.config.ts
index 24929e2..7bd6e2a 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -1,4 +1,5 @@
 import { cosyteVitest } from "@cosyte/vitest-config";
+import { configDefaults } from "vitest/config";
 
 /**
  * Vitest config for @cosyte/ccda from the shared @cosyte/vitest-config standard.
@@ -35,12 +36,51 @@ import { cosyteVitest } from "@cosyte/vitest-config";
  * The measurement that sized them is in the `CHANGELOG.md` entry for
  * `PARSER-TESTTIMEOUT-ASSERTS-AN-IDLE-BOX`, dated, where it cannot drift out from under this file.
  * Re-measure rather than inherit it, and do not put a number back here.
+ *
+ * **THE WORKING-TREE SNAPSHOT RUNS ALONE, AFTER EVERY OTHER FILE, AND THAT IS WHAT MAKES IT A
+ * MEASUREMENT.** `test/conformance/working-tree-is-clean.test.ts` takes `git status` of the real
+ * working tree before and after a conformance run and requires the two sets to be equal. Vitest runs
+ * test files in parallel, and other files write into that same tree for a moment: the README usage
+ * case in `test/docs-content.test.ts` writes `.cosyte-usage-stdout/` and removes it in `afterAll`, the
+ * `pnpm build` it spawns has `tsup` write and delete `tsup.config.bundled_<hash>.mjs` at the root, and
+ * the override-gate case in `test/scripts/phi-scan.test.ts` appends to the tracked
+ * `phi-scan-overrides.md` and restores it. A snapshot taken inside one of those windows compares
+ * another file's leftovers, not the run's. The release Verify step on `d906bda` went red that way,
+ * with `.cosyte-usage-stdout/` in the "before" set and gone from the "after" one. So the file has a
+ * project of its own, with a later `sequence.groupOrder`: Vitest starts that group only after the
+ * group before it has finished, so nothing else is writing while the snapshot is taken. The
+ * assertion itself is unchanged. Do not answer a red from it by loosening the comparison or by
+ * moving the run into a temporary directory. A new test that has to see the tree while nothing
+ * else writes to it joins the second project; every other file stays in the first, which still
+ * runs in parallel.
  */
+const WORKING_TREE_SNAPSHOT = "test/conformance/working-tree-is-clean.test.ts";
+
 export default cosyteVitest({
   coverageDirs: ["parser", "model", "model/types", "helpers", "serialize", "profiles", "builder", "edit"],
   test: {
     globals: false,
     environment: "node",
-    include: ["test/**/*.test.ts", "src/**/*.test.ts"],
+    // `extends: true` CONCATENATES `include` rather than replacing it, so the globs live on the
+    // first project only: set at the root, they would pull the whole suite into the second one.
+    projects: [
+      {
+        extends: true,
+        test: {
+          name: "suite",
+          include: ["test/**/*.test.ts", "src/**/*.test.ts"],
+          exclude: [...configDefaults.exclude, WORKING_TREE_SNAPSHOT],
+          sequence: { groupOrder: 0 },
+        },
+      },
+      {
+        extends: true,
+        test: {
+          name: "working-tree",
+          include: [WORKING_TREE_SNAPSHOT],
+          sequence: { groupOrder: 1 },
+        },
+      },
+    ],
   },
 });